Widespread natural methane and oil leakage from sub-marine Arctic reservoirs

Parceling the anthropogenic and natural (geological) sources of fossil methane in the atmosphere remains problematic due to a lack of distinctive chemical markers for their discrimination. In this light, understanding the distribution and contribution of potential geological methane sources is important. Here we present empirical observations of hitherto undocumented, widespread and extensive methane and oil release from geological reservoirs to the Arctic Ocean. Methane fluxes from >7000 seeps significantly deplete in seawater, but nevertheless reach the sea surface and may transfer to the air. Oil slick emission spots and gas ebullition are persistent across multi-year observations and correlate to formerly glaciated geological structures, which have experienced km-scale glacial erosion that has left hydrocarbon reservoirs partially uncapped since the last deglaciation ~15,000 years ago. Such persistent, geologically controlled, natural hydrocarbon release may be characteristic of formerly glaciated hydrocarbon-bearing basins which are common across polar continental shelves, and could represent an underestimated source of natural fossil methane within the global carbon cycle.

I understand that this may partly be out of the scope of the current study, but as a reader I would be interested to learn more if anything can be said about both the fraction of the seeped methane reaching the atmosphere (and absolute amount). As a reader, I would be also interested to be provided with some information (maybe in the methods) if the authors considered the potential effect of tides (it has been found in the past that seepage might relate to tidal fluctuations at some locations whereas not at others; this might not be assessable for the current data set, but it is a question if such transient effects might potentially have affected the observed seapage distribution (was it important whether measurements at a given location were made at low versus high tide?)). Finally, because surface areas and number of seeps are provided, more information about what these quantities are would be warranted (e.g., investigated surface area (investigated with instruments) versus overall surface area of the region); the question also arises when the current authors compare a number of seeps they detected within a given surface area with number of seeps detected by previous authors within given surface areas: are the numbers really comparable(?). However, I do understand that the current study is already very detailed and focused on mapping the overall distribution of the seepage--the level of detail provided seems already appropriate, and these aspects might be discussed only very shortly (if at all), if appropriate/doable. Additionally, I have a few more minor, line-by-line comments: -line 23: would "widespread" be more accurate than "pervasive"? -line 36: maybe remove "fugitive"; it is my understanding that some leakages may not be transient, as the use of this word might imply. -line 37: consider adding "natural" before seepage? -line 53: "contains" to "contain". -line 98: add unit for " 940 to 1180". -line 117 and throughout the manuscript: it would help if the authors could clarify what they mean with the surface areas provided. In a general area of X km2, only Y km2 were covered by the multibeam survey. This is clear from Figure 3, which also indicates that ovelap happened (i.e., the authors measured several times at the same location). It would help to clarify a little these aspects, in the main text or in the methods. -line 146: figure 3: weak, medium, and strong flares are difficult to differentiate when many flares overlap (points of same color but different sizes). The "high flares" are very visible, and this is maybe on purpose that the authors desired to focus the reader's attention on those.
-line 175 " intermediate water interval"; it may help to explain how the different intervals were defined. -lines 180-181. "heavier methane homologs" is unclear. Do the authors refer to C2-C5 hydrocarbons (ethane, propane, etc.), or something similar? I am not used to the terms "methane homologs". -lines 304-307: this effect is unlikely to be significant. As highlighted by the authors's figure 5d, the methane concentrations they observed remained much lower that the methane saturation aqueous solubility. I.e., the ambient methane concentrations are unlikely to substantially decrease the rate of aqueous dissolution of a gas bubble composed of initially close to 100% methane gas. This is actually what is stated by the also argued that it is unlikely to happen anywhere in the global oceans that concentrations large enough to substantially slow down aqueous dissolution of methane bubbles would be observed. And the current authors do not seem to show any concentration in the water column that would show that the analysis by Gros et al. was inaccurate.) [Whereas reduced rate of mass transfer is unlikely to have a substantial effect at concentrations reported to date in seawater worldwide (as far as I am aware of), it is possible that large discharges of gas bubbles result in multiphase plumes (constituted of gas bubbles and entrained seawater), such that the overall ascent speed towards the sea surface is increased. This might need to be evaluated and might be a more significant effect than the effect described by the authors on aqueous dissolution rate.] -line 311: this may be difficult in absence of measurement of bubble size, but discussion of the extent of methane emitted at the sea surface might have been of interest (see e.g. McGinnis et al., 2006, "Fate of rising methane bubbles in stratified waters: How much methane reaches the atmosphere?").Though I understand that this might not be available with the data collected during this study. And the authors already give some information e.g. by their re-analysis of the measurements by Platt et al. (around line 315).
-line 323: "our methane concentration data collected within oil slicks"; unclear: are these concentration in water immediately below these slicks? Or in air? Or in the water at deeper water depth (which water depth)?
Reviewer #2 (Remarks to the Author): The manuscript "Widespread natural methane and oil leakage rom sub-marine Arctic reservoirs" presents a comprehensive analysis of seismic data, water column echosounder images of gas seepage, methane concentration in seawater from Norwegian Barents Sea, Arctic Ocean indicting an underestimated sources of methane and oil from hydrocarbon reservoirs to the ocean and possibly to the atmosphere. The respond of hydrocarbon reservoirs to environmental changes (especially deglaciation) is of great relevance to predict feedback mechanisms during present-day global warming and thus highly relevant. The style and language of the manuscript is of high quality and the text is easy to follow. I have one major comment, which I would like to be addressed by the authors. The main premise of the study is that all methane to the seawater originates from leaking petroleum-bearing sediments. One could assume that physical transport of methane to the surface may be the primary pathway; one could not rule out the local methanogenesis of the sea water. Multiple recent studies have repeatedly shown, methane production can and does occur under both anoxic and oxic conditions in the seas. Areas with widespread methane and oil leakage make such local methanogenesis even more likely. I wonder if the geochemical data of the methane in sea water is available, which is useful for the discrimination.

Point-by-point response
Reviewer #1: I understand that this may partly be out of the scope of the current study, but as a reader I would be interested to learn more if anything can be said about both the fraction of the seeped methane reaching the atmosphere (and absolute amount) Calculating the total amount of methane reaching the atmosphere at our study area relying on few discrete measurements alone is problematic because neither the size of the methane plume in the water is known, nor the transient concentration gradients within it. All our water samples collected within 14 x 9 km rectangular area happen to be taken from one such plume as they demonstrate 19% to 35% CH4 supersaturation. For comparison, a methane flux survey of Silyakova et al., 2020 2 contained 64 sampling stations within 22 x 10 km study site, which were repeated in 3 consecutive years. Given that the most active seep region in Sentralbanken spans over at least 620 km 2 , more dense and repeated measurements of methane concentrations in the water are necessary. Our group has collected and will be collecting more water column and air samples, which will be used in a different paper.
Reviewer #1: "As a reader, I would be also interested to be provided with some information (maybe in the methods) if the authors considered the potential effect of tides (it has been found in the past that seepage might relate to tidal fluctuations at some locations whereas not at others; this might not be assessable for the current data set, but it is a question if such transient effects might potentially have affected the observed seapage distribution (was it important whether measurements at a given location were made at low versus high tide?))" Authors: Indeed, tides cause reciprocal hydrostatic pressure perturbations in the shallow subsurface. Such fluctuations affect pressure-dependent solubility of gases in pore waters and may cause solution and exsolution of methane vapour phase. As Reviewer 1 points out, the strength of tidal effects on methane seepage changes significantly across seepage regions worldwide. This is likely due to the variable abundance of methane in pore water and variable solubility limits dictated by water depth and subseafloor temperature at each region. If the amount of methane in subseafloor is at or close to the solubility limit, small pressure decrease due to low tide may cause free gas release from otherwise ebullition-free seafloor 3 . However, if methane vapour phase is available within the subseafloor in abundance and the ebullition is ongoing (e.g. at high tide), additional degassing at low tide is likely to be less noticeable on echosounder data. Our analyses show that in three areas the number of flares increases at lower tide, in the three areas the number of flares decreases at lower tide, and in one area the number of flares remains the same regardless of tide settings. Thus, we do not observe a correlation between tide fluctuations and seep activity. We suggest that a potent thermogenic hydrocarbon sources at our study sites provide sustained and extensive gas flux through the seafloor significantly exceeding methane solubility limits and, thus, overriding detectable tidal solution-exsolution transient effects.
Reviewer #1: "the question also arises when the current authors compare a number of seeps they detected within a given surface area with number of seeps detected by previous authors within given surface areas: are the numbers really comparable(?). However, I do understand that the current study is already very detailed and focused on mapping the overall distribution of the seepage--the level of detail provided seems already appropriate, and these aspects might be discussed only very shortly Reviewer #1:-line 23: would "widespread" be more accurate than "pervasive"?

Authors: done
Reviewer #1:-line 36: maybe remove "fugitive"; it is my understanding that some leakages may not be transient, as the use of this word might imply.

Authors: done
Reviewer #1:-line 117 and throughout the manuscript: it would help if the authors could clarify what they mean with the surface areas provided. In a general area of X km2, only Y km2 were covered by the multibeam survey. This is clear from Figure 3, which also indicates that ovelap happened (i.e., the authors measured several times at the same location). It would help to clarify a little these aspects, in the main text or in the methods.
Authors: Thank you for pointing this out. Done.
Reviewer #1:-line 146: figure 3: weak, medium, and strong flares are difficult to differentiate when many flares overlap (points of same color but different sizes). The "high flares" are very visible, and this is maybe on purpose that the authors desired to focus the reader's attention on those.
Authors: due to the high density of flares, it is difficult to avoid overlapping symbols indicating flares. That is why we provided flare density maps (Figure 4). With Figure 3 we aim to show that the flares are abundant, and their types are mixed within the clusters (there are no flare clusters composted of only strong flares or only weak layers). "High flares" were purposely placed as the overlying map layer to make sure they are well visible as we specifically discuss them in the text. [Whereas reduced rate of mass transfer is unlikely to have a substantial effect at concentrations reported to date in seawater worldwide (as far as I am aware of), it is possible that large discharges of gas bubbles result in multiphase plumes (constituted of gas bubbles and entrained seawater), such that the overall ascent speed towards the sea surface is increased. This might need to be evaluated and might be a more significant effect than the effect described by the authors on aqueous dissolution rate.] Authors: thank you for pointing this out. We revisited Gros et al., paper and agree on the Reviewer 1 comment. We removed the discussion of the reduced mass transfer between the bubbles and the water due to somewhat elevated dissolved methane content. Authors: Bubble size measurements are unfortunately not available. Nevertheless, we added a few sentences on the fate of methane bubbles in the water column (e.g. mass transfer between the gas bubbles and the surrounding water) (lines 337-340).
Reviewer #1:-line 323: "our methane concentration data collected within oil slicks"; unclear: are these concentration in water immediately below these slicks? Or in air? Or in the water at deeper water depth (which water depth)?
Authors: It was collected 5 m below the slicks. We clarified it in line 334.
Reviewer #2: "I have one major comment, which I would like to be addressed by the authors.
The main premise of the study is that all methane to the seawater originates from leaking petroleumbearing sediments. One could assume that physical transport of methane to the surface may be the primary pathway; one could not rule out the local methanogenesis of the sea water. Multiple recent studies have repeatedly shown, methane production can and does occur under both anoxic and oxic conditions in the seas. Areas with widespread methane and oil leakage make such local methanogenesis even more likely. I wonder if the geochemical data of the methane in sea water is available, which is useful for the discrimination." Authors: Indeed, some methanogens have been shown tolerant to oxygen and in vitro studies proposed biochemical pathways of methanogenesis in sea water 11,12 . However, understanding whether these pathways may generate sizeable sea water methane plumes is currently lacking. The majority of studies concerning methanogenesis in oxic conditions has been carried out in lacustrine settings.
Discriminating methane generated in the sea water at our study site requires analysis of isotopic composition of carbon and hydrogen of dissolved methane coupled with nutrient analysis and biogeochemical experiments of isolated samples. Unfortunately, we do not have equipment, expertise and sample material to carry out such biogeochemical investigations. Given lack of geochemical data to discriminate potential methane production in oxic conditions at our study sites, we agree with Reviewer 2 that we cannot rule out a possibility that a fraction of gas dissolved in the water at Sentralbanken area may originate from local methanogenesis in sea water (lines 185-187) Methanogenesis in sea water can be ruled out as a contributor to free gas phase with certainty because low rates of local methane production (a range between 0.04 and 0.23 mmol m -3 day -1 has been reported for lake environments 11,13 ) are incompatible with generating methane at quantities required to form gas bubbles.